1. Field of Invention
The present invention relates generally to the field of data communications. More specifically, the present invention relates to estimating transmission channel data carrying capacity.
2. Description of the Related Art
A conventional telephone system is described with respect to FIG. 1. A central office switch 102 is connected to subscribers 104a-104e via cable pairs 106a-106e respectively. Generally, cable pairs 106a-106e are part of a larger cable that contains many cable pairs, for example, having 300, 600, 1200 or 2400 cable pairs. Cable pairs 106a-106e are also called local loops as they make a loop from the central office to the subscriber and back to the central office.
Conventional local loops are designed to carry telephone traffic in the plain old telephone system (POTS). Consequently, the loops were designed for traffic having frequencies up to approximately 4 KHz. More recent data transmission schemes carry data having significantly higher frequencies. For example, communicating using asymmetric digital subscriber line (ADSL) requires data rates ranging from 25 KHz to 1.1 MHz. As described below, some, but not all, loops in the existing infrastructure can be used to transmit these higher rate data streams.
An additional limitation to the data carrying capacity of some cable pairs is the presence of load coils. Load coils, such as load coils 108a-108c, are inductors spliced into a cable pair along its length to mitigate the effects of cable length on data transmission. Load coils are present only on longer cable pairs. Where load coils are present, however, they limit the pass band of the cable to which they are attached. This is shown in FIG. 2. FIG. 2 illustrates a transfer function (curve 202) representative of a cable having no added load coils. As can be seen, the gradual roll off of the transfer function indicates that higher frequencies are passed by the cable, including frequencies required for higher data rate transmission schemes, including, for example, ADSL.
This gradual rolloff is in sharp contrast to a cable to which load coils have been added, as illustrated by the exemplary transfer function (curve 204) of a cable to which load coils have been added. It can be seen that the transfer function has a sharp cutoff at approximately 3.3 KHz. Consequently, cable pairs having load coils spliced in them cannot be used to carry higher data rate transmissions such as ADSL. Because not all local loops can be used for ADSL, or other high data rate transmission services, when a customer requests ADSL service, the service provider must determine whether the existing lines to the customer can support the relatively high bandwidth requirements of ADSL.
The problem of determining the sufficiency of a local loop's bandwidth, and particularly, whether the local loop has load coils spliced into it, is significant in the industry. Originally, paper records tracked which local loops had load coils. However, today these records are often incomplete, lost or unreliable or otherwise not easily accessible. Thus, they tend to be of little use in responding to a customer's request for high data rate services. Consequently, service technicians must often go to a subscriber's home to test a cable pair to determine if the cable pair can support high data rate services. Sending service technicians to subscriber sites is costly, inconvenient and time consuming.
Conventionally, the presence of load coils is detected by hooking up a test set across the tip and ring of the cable pair to detect the presence of load coils. This test is illustrated in FIGS. 3A to 3C. FIG. 3A shows a non-loaded cable pair. A non-loaded cable pair 302 can be characterized as a series of capacitances 302a, 302b and 302c in parallel as shown in FIG. 3A. The capacitance shown in FIG. 3A is that capacitance between the conductors, which is carefully controlled in the manufacturing process to be within certain limits. The length and gauge of the a particular cable segment gives rise to the capacitance. Curve 320 in FIG. 3C illustrates graphically a representative dependency of the impedance on frequency for cable pair 302. From the Figure, it can be seen that the dependency has a form characteristic of a capacitor. This is because as the frequency increases, the impedance decreases and asymptotically approaches the characteristic impedance of the cable. In the present example, the characteristic impedance is 100 ohms, shown by line 321.
A loaded pair, on the other hand, includes inductors 304a and 304b (load coils) in series as shown in FIG. 3B, in addition to capacitances 302a, 302b and 302c. Curve 322 in FIG. 3C illustrates graphically a representative dependency of impedance on frequency for a local loop with a single load coil. There is a peak 324 in curve 322. Peak 324 is present due to a tuned circuit that arises from the inductor-capacitor combinations, e.g., inductor 304a and capacitor 302b. The frequency at which the apex of the peak occurs is called the resonant frequency. The resonant frequency is the frequency at which the circuit is said to be “tuned.” Thus, the presence of load coils can be determined by detecting the presence of a tuned circuit in the cable pair.